1. Field of the Invention
This invention relates to an optical disc recording apparatus adapted for processing input data digitized from continuous, i.e. analog signals with compression and subsequently recording the resulting bit compressed data on an optical disc.
2. Description of Related Art
An optical disc may have a recording capacity larger by two or three digits than that of a magnetic disc, while enabling accessing at a higher speed than that for a tape recording medium. The optical disc also has an advantage that it enables contactless data recording/reproduction on or from the medium, and is superior in durability, so that it has become more popular in recent years. A so-called compact disc (CD) is among the widely known types of the optical discs.
Meanwhile, for providing a portable and, above all, a pocketable headphone stereo or similar recording/reproducing apparatus, with the use of the optical disc, a compact disc with a disc diameter of 12 cm and a compact disc with a disc diameter of 8 cm (so-called CD single) have been prescribed, as for as the disc format is concerned. However, with the disc diameter of 12 cm, the recording/reproducing apparatus becomes too bulky to be portable. Therefore, a disc 8 cm or less in diameter may be thought to be convenient. However, if it is attempted to construct a portable or pocket size recording and/or reproducing apparatus with the use of an optical disc 8 cm or less in diameter, the following problem is raised.
In the case of a standard CD format, in which an optical disc, on which are recorded stereophonic digital PCM audio signals sampled with a sampling frequency of 44.1 kHz and quantized by 16 bits, is supplied by a producer, and in which these signals are exclusively reproduced by the user (CD-DA format), the playback time (recording time) of the disc which is 8 cm in diameter is 20 to 22 minutes at most, meaning that a symphony of classical music can not be recorded completely on one disc side. A playback time of 74 minutes or longer, which is approximately equal to that of the compact disc 12 cm in diameter, is desired. In addition, with this CD-DA format, recording by the user is not feasible. In addition, a contactless type optical pickup device is vulnerable to mechanical vibrations and subject to detracking or defocusing. Thus, when the apparatus is to be portable, some strong measures need to be taken to prevent adverse effects of detracking or defocusing on the reproducing operation.
With the CD-I (CD-interactive) format, the levels A to C as shown in the following Table 1 are prescribed as modes for recording/reproducing bit-compressed digital audio signals.
TABLE 1 ______________________________________ number of bits for playback time sampling quantiza- (stereo/ levels frequency tion bandwidth monaural) ______________________________________ A 37.8 kHz 8 17 kHz 2/4 B 37.8 kHz 4 17 kHz 4/8 C 18.9 kHz 4 8.5 kHz 8/16 ______________________________________
When reproducing a disc recorded with, for example, the level B mode, signals obtained by fourfold bit compression of standard CD-DA format digital signals are reproduced. Thus, if all of the recorded data are stereophonic audio compressed data, playback time becomes fourfold, or reproduction of four-channel data, becomes feasible, so that reproduction for 70 minutes or longer becomes possible with an optical disc of the order of 6 cm in diameter.
Meanwhile, with the CD-I format, the disc is rotationally driven at the same linear velocity as that for the standard CD-DA format, so that the continuous audio compressed data are reproduced at a rate of one unit to n recorded units on the disc where n is a figure corresponding to the playback time or the bit compression rate of data and is equal to 4(n=4) with the level B stereo mode. This unit is termed a block or sector which is made up of 98 frames and has a period of 1/75 second. Therefore, with this level B stereo mode, a data string in which one of four sectors is an audio sector, such as EQU S D D D S D D D . . .
where S is an audio sector and D is other data sector or sectors, is recorded on the sector-by-sector basis on the disc. However, for actual recording, since the above data string undergoes a predetermined encoding similar to that for ordinary CD format audio data, such as error correction coding and interleaving, data of the audio sector S and data of the data sector D are arranged in a scrambled fashion in the recording sectors on the disc. The other data sectors D may for example be video or computer data. When the bit-compressed audio signals are also used for the data sector D, a data string in which four-channel audio sectors S1 to S4 are cyclically arranged, that is a data string EQU S1 S2 S3 S4 S1 S2 S3 S4 . . .
is encoded and recorded on the disc. When recording and reproducing continuous audio signals, channel 1 data corresponding to the audio sector S1 are reproduced from the innermost to the outermost sides of the disc. Channel 2 data corresponding to the audio sector S2 are reproduced from the innermost to the outermost sides of the disc. Channel 3 data corresponding to the next audio sector S3 are reproduced from the innermost to the outermost sides of the disc. Finally, channel 4 data corresponding to the audio sector S4 are reproduced from the innermost to the outermost sides of the disc to enable data reproduction for a continuous fourfold time duration.
However, for the above mentioned continuous reproduction, several track jump operations of a long distance spanning the inner and outer disc peripheries are necessitated. Since the track jump can not be achieved instantaneously, playback data become depleted for a short time period, meaning that the playback sound is momentarily interrupted. On the other hand, when continuous audio signals are to be recorded, it is not possible to record only the sector 2 signals, as an example, because the data need to undergo interleaving at the time of recording. That is, sector 2 data need to be interleaved with adjoining and even near-by sectors, such as sectors S1 and S3, such that it becomes necessary to rewrite signals of previously recorded sectors. Thus it is extremely difficult to record the continuous compressed audio data, while real-time processing is virtually impossible.